The V8 Owners Forum -

Hello, I know this has proberly been done before but im getting confussed!
Im building a 4litre engine, it should be a 9.35-1 compression, i am using com gaskets and 28cc heads which are being ported and having bigger valves. As far as i can see the heads havent been skimmed before, so how much needs to be removed to get a compression of 10.5-1?
Its having a cam change, rhoads lifters, a holley, mallory with MSD.

Thanks in advance.

Paul V8 wrote:Hello, I know this has proberly been done before but im getting confussed!
Im building a 4litre engine, it should be a 9.35-1 compression, i am using com gaskets and 28cc heads which are being ported and having bigger valves. As far as i can see the heads havent been skimmed before, so how much needs to be removed to get a compression of 10.5-1?
Its having a cam change, rhoads lifters, a holley, mallory with MSD.

Thanks in advance.

http://www.wallaceracing.com/cr_test2.php

I will estimate approx 1mm?

It is better to skim block if possible?, you will get better quench then = more power

OK first off I would not push the CR up to 10.5:1 whilst just relying on Rovers figures of 28cc and their stated CR of 9.35:1. I think that you are well into the stage where you should be measuring everything with a burette.

Having said that if you want to risk it here are the figures:-

Each cylinder is 493cc
Each chamber is 28cc
The gasket is 8.3cc in volume (If it is 1.2mm thick...I don't know how thick they actually are!)
The piston dish is 22.7cc

In order to have a CR of 9.35:1 there must be a total of 59cc above the piston at TDC.

CR= ([cyl vol] + [total vol above the piston]) / [total vol above the piston]

In order to get a CR of 10.5:1 the vol above the piston needs to be 52cc, that is a reduction of 7cc.

Each 8 thou off the heads is worth 1 cc out of the chambers, therefore 56 thou needs to come off the heads. (8x7)

Bear in mind that 8 thou off the BLOCK will take more than 1cc off the volume. The chambers are not circular, the bores are.

Like I said, there is no way that I'd do this just going on the gumph that I've posted above!

I measured a 4,6 litre engine just for fun when I took it apart and it was not even near the factory quoted CR. So, don´t assume that it is 9.35 even if the factory says it is. Measure the actual CR and determine how much you need to machine.

minorv8 wrote:I measured a 4,6 litre engine just for fun when I took it apart and it was not even near the factory quoted CR. So, don´t assume that it is 9.35 even if the factory says it is. Measure the actual CR and determine how much you need to machine.

Yep, that's the problem, the calculations above are using the 9.35:1 figure in order to work out the volume above the piston at TDC. If the stated CR is wrong then it messes up the calulations.

It is a messy time consuming job to measure all of the chambers and all of the pistons but then again so is rebuilding a blown up motor!

minorv8 wrote:I measured a 4,6 litre engine just for fun when I took it apart and it was not even near the factory quoted CR. So, don´t assume that it is 9.35 even if the factory says it is. Measure the actual CR and determine how much you need to machine.

Hi,
was the CR more than 9.35 or less than that?
Many thanx
Stephan

Less.